1. Field of The Invention
The present invention relates to soldering material, more particularly soldering material which is used for soldering parts which are used under environments where fatigue is likely to occur, and particularly for soldering electronic parts on a printed circuit substrate.
2. Description of Related Arts
Generally, the basic components of soldering material are binary Sn-Pb. It is known that various components are added so as to improve the properties of the binary Sn-Pb alloy.
Japanese Examined Patent Publication No. 40-25885 discloses the fact that copper, silver, nickel or the like have been added to the soldering material in order to prevent the copper of an electric soldering-iron from dissolving into the soldering material, and thus damaging it. It is described therein that damage is prevented through the function of silver or nickel which leads to the uniform dispersion of copper.
Japanese Examined Patent Publication No. 45-2093 discloses the fact that corrosion-resistance of solder with aluminum alloy at the soldered part is improved by addition of Ag or Sb, and, further, fluidity and ease of the soldering operation are improved by addition of Cd.
The following is a prior art which intends to improve specifically the soldering materials used for integrated circuits and printing substrates.
Japanese Examined Patent Publication No. 52-20377 discloses concurrent addition of Cu and Ag, in order to prevent a thin copper-wire which is to be soldered, from being dissolved and eroded by solder, thereby incurring a strength-reduction. According to the description, Cu suppresses the soldering material from encroaching on the material to be soldered. However Cu, when added, raises the melting point of the soldering material, with the result that the melting of the material to be soldered is likely to occur. The liability of melting can be prevented by Ag which has the effect of lowering the melting point.
Japanese Unexamined Patent Publication No. 56-144893 aims to eliminate the drawback that the silver in a silver lead-wire of a ceramic capacitor, diffuses into the soldering material, thereby deteriorating the characteristics of the capacitor or peeling the silver surface. The above mentioned publications also aims to enable high-speed soldering and proposes Sn-Sb-Ag-Pb soldering material.
Japanese Unexamined Patent Publication No. 59-70490 proposes a 1-15% Sb-1-65% Sn(In)-Pb component and a Sb-Ag-Sn(In)-Pb component.
Regarding the properties of soldering material used for soldering electronic parts moved on an integrated circuit or a printed circuit, recently, attention is paid to the failure in current conduction which occurs because cracks are generated in the solder which bonds a lead-wire to a rand of a printed substrate. Presumably, the reason is as follows: stress is generated in a substrate and mounted electronic parts due to periodic change in the temperature in which they are used; the bonding member, i.e., the solder, undergoes this stress and is hence exposed constantly to stress; therefore, fatigue fracture occurs after long time use. In addition, the following facts seem to be reasons for accelerating the fatigue fracture: temperature rises at the soldered parts due to current conduction; heat generation occurs in the electronic parts; and, a printed circuit is subjected to mechanical vibration.
It has been clarified that the binary Sn-Pb soldering material consisting of the basic components involves a problem in fatigue resistance when it is used in an environment where it is exposed to thermal and mechanical stress for long period of time. Although fatigue resistance has heretofore been extensively studied with regard to materials and welded sections of aluminum and steel, methods for improving fatigue resistance of soldering material having not yet been clarified.